The objective of this study was to evaluate the effect of the tine shape of 3 different periodontal probes. One tapered, one parallel-sided and one WHO-probe tine, each with a diameter of 0.5 mm at the tip, were mounted in hinged handles exerting a constant probing force (Brodontic). The handles were adjusted to either 0.25 N (127 N/cm2) or 0.5 N (255 N/cm2). 12 patients with moderate to severe periodontitis were measured after supra- and subgingival debridement, using all 6 possible tine/force combinations in 3 sessions. In each session one tine/force combination was used in the 1st and 3rd quadrants, and another tine/force combination in the 2nd and 4th quadrant. The measurements in the same quadrants could therefore be used for comparisons within the same site. The selection for the 2 quadrants in which a given tine/force combination was to be used, was randomised. Calculations of differences (mean per patient) between probing measurements show that the WHO tine yields deeper recording than the parallel/sided and tapered tines, both at 127 N/cm2 and 255 N/cm2. We conclude that in addition to probing force, the tine shape of a periodontal probe is of significant importance for the recorded probing depth.